SOUND

ow do you come to know that a

‘period’ is over in your school?

You come to know easily that

someone is at your door when he knocks

or you hear the sound of the doorbell.

Most of the time you can make out that

someone is approaching you by just

hearing the foot steps.

You might have played a game called

hide and seek. In this game a person is

blind-folded and has to catch the

remaining players. How is the blind-

folded person able to guess which player

is closest to her?

Sound plays an important role in our

lives. It helps us to communicate with

one another. We hear a variety of sounds

in our surroundings.

Make a list of sounds you hear in

your surroundings.

In the music room of your school you

hear the sounds produced by musical

instruments like flute, tabla,

harmonium, etc. (Fig. 10.1).

How is sound produced? How does

travel from one place to another? How

do we hear sound? Why are some sounds

louder than others? We shall discuss

such questions in this chapter.

Fig. 10.1 : Some musical instruments

Tabla

Harmonium

Sitar

Flute

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10.1 Sound is Produced by a

Vibrating Body

Touch the school bell when not in use.

What do you feel? Again touch it when

producing sound. Can you feel it

vibrating?

Activity 10.1

Take a metal plate (or a pan). Hang

it at a convenient place in such a

way that it does not touch any wall.

Now strike it with a stick (Fig.10.2).

Do you hear a sound? Touch the

plate or pan gently with your finger.

Do you feel the vibrations?

Fig. 10.2 : Striking a pan

Again strike the plate with the stick

and hold it tightly with your hands

immediately after striking. Do you

still hear the sound? Touch the

plate after it stops producing sound.

Can you feel the vibrations now?

As you learnt in Class VII the to and

fro or back and forth motion of an

object is called vibration. When a

tightly stretched band is plucked, it

vibrates and produces sound. When it

stops vibrating, it does not produce

any sound.

Activity 10.3

Take a metal dish. Pour water in it.

Strike it at its edge with a spoon

(Fig. 13.4). Do you hear a sound?

Again strike the dish and then

touch it. Can you feel the dish

vibrating? Strike the dish again.

Look at the surface of water. Do you

see any waves there? Now hold the

dish. What change do you observe

on the surface of water? Can you

explain the change? Is there a hint

to connect sound with the

vibrations of a body?

Activity 10.2

Take a rubber band. Put it around

the longer side of a pencil box

(Fig. 10.3). Insert two pencils

between the box and the stretched

rubber. Now, pluck the rubber band

somewhere in the middle. Do you

hear any sound? Does the band

vibrate?

Fig. 10.3 : Plucking the rubber band

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125

We see that a vibrating object

produces sound. In some cases, the

vibrations are easily visible to us. But

in most cases, their amplitude is so small

that we cannot see them. However, we

can feel them.

Activity 10.4

Take a hollow coconut shell and

make a musical instrument ektara.

You can also make it with the help

of an earthen pot (Fig. 10.5). Play

this instrument and identify its

vibrating part.

Table 10.1 : Musical Instruments and

their Vibrating Parts

S.No. Musical Vibrating Part

Instrument Producing Sound

1. Veena Stretched string

2. Tabla Stretched

membrane

Many of you might have seen the

manjira (cymbals), the ghatam, and the

noot (mudpots) and the kartal. These

musical instruments are commonly

used in many parts of our country. These

instruments are simply beaten or struck

(Fig. 10.6). Can you name a few other

musical instruments of this type?

You too can make a musical

instrument.

Fig. 10.4 :

Vibrating dish produces waves

in water

Fig. 10.6 :

A few more musical instruments

Ghatam

Make a list of familiar musical

instruments and identify their vibrating

parts. A few examples are given in

Table 10.1. Complete rest of the Table.

Fig. 10.5

: Ektara

Manjira

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10.2 Sound Produced by

Humans

Speak loudly for a while or sing a song,

or buzz like a bee. Put your hand on

your throat as shown in Fig. 10.8. Do

you feel any vibrations?

In humans, the sound is produced

by the voice box or the larynx. Put your

fingers on the throat and find a hard

bump that seems to move when you

swallow. This part of the body is known

as the voice box. It is at the upper end

of the windpipe. Two vocal cords, are

stretched across the voice box or larynx

in such a way that it leaves a narrow

slit between them for the passage of air

(Fig.10.8).

Fig. 10.7 : Jaltrang

When we pluck the string of an

instrument, like the sitar, the sound

that we hear is not only that of the

string. The whole instrument is forced

to vibrate, and it is the sound of the

vibration of the instrument that we

hear. Similarly, when we strike the

membrane of a mridangam, the sound

that we hear is not only that of the

membrane but of the whole body of

the instrument.

When we speak, does

any part of our body

vibrate?

Fig.10.8 : Voice box in humans

When the lungs force air through the

slit, the vocal cords vibrate, producing

sound. Muscles attached to the vocal

cords can make the cords tight or loose.

When the vocal cords are tight and thin,

the type or quality of voice is different

Activity 10.5

Take 6-8 bowls or tumblers. Fill

them with water up to different

levels, increasing gradually from

one end to the other. Now take

a pencil and strike the bowls gently.

Strike all of them in succession. You

will hear pleasant sounds. This is

your jaltrang (Fig.10.7).

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from that when they are loose and

thick. Let us see how the vocal cords

function.

Activity 10.6

Take two rubber strips of the same

size. Place these two pieces one

above the other and stretch them

tight. Now blow air through the gap

between them [Fig. 10.9(a)]. As the

air blows through the stretched

rubber strips, a sound is produced.

You can also take a piece of paper

with a narrow slit and hold it

between your fingers as shown in

Fig. 10.9 (b). Now blow through the

slit and listen to the sound. Our

vocal cords produce sound in a

similar manner.

(a)

(b)

Fig. 10.9 (a), (b) : Working of vocal cords

The vocal cords in men are about

20 mm long. In women these are about

15mm long. Children have very short

vocal cords. This is the reason why

the voices of men, women and

children are different.

10.3 Sound Needs a Medium

for Propagation

When you call up your friend who is

standing at a distance, your friend is

able to hear your voice. How does the

sound propagate or travel to her?

Activity 10.7

Take a metal or glass tumbler. Make

sure that it is dry. Place a cell phone

in it. (Remember that the cell phone

must not be kept in water.) Ask your

friend to give a ring on this cell

phone from another cell phone.

Listen to the ring carefully.

Now, surround the rim of the

tumbler with your hands

(Fig. 10.10). Put your mouth on the

Fig. 10.10 : Sound needs a medium to travel

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Place your ear gently on the water

surface (Fig. 10.11). (Be careful : the

water should not enter in your ear.)

Can you hear the sound of the bell?

Does it indicate that sound can

travel through liquids?

opening between your hands.

Indicate to your friend to give a ring

again. Listen to the ring while

sucking air from the tumbler.

Does the sound become fainter

as you suck air?

Remove the tumbler from your

mouth. Does the sound become

loud again?

Can you think of an explanation? Is

it possible that the decreasing amount

of air in the tumbler had something to

do with decreasing loudness of the ring?

Indeed, if you had been able to suck

all the air in the tumbler, you will not

listen any sound. Actually, sound needs

a medium to travel. When air has been

removed completely from a vessel, it is said

that there is a vacuum in the vessel. The

sound cannot travel through a vacuum.

Does sound travel in liquids? Let us

find out.

Activity 10.8

Take a bucket or a bathtub. Fill it

with clean water. Take a small bell

in one hand. Shake this bell inside

the water to produce sound. Make

sure that the bell does not touch

the body of the bucket or the tub.

Oh ! That is how whales

and dolphins might be

communicating under

water.

Fig. 10.11 : Sound travelling through water

Fig. 10.12 : Sound travelling through a

metre scale

Can you hear the sound of the

scratching? Ask your friends

around you if they were able to hear

the same sound?

Let us find out if sound can travel

through solids also.

Activity 10.9

Take a metre scale or a long metal

rod and hold its one end to your

ear. Ask your friend to gently

scratch or tap at the other end of

the scale (Fig. 10.12).

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129

You can also perform the above

activity by placing your ear at one end

of a long wooden or metallic table and

asking your friend to gently scratch the

other end of the table (Fig. 10.13).

Fig. 10.13 : Sound can travel through solids

We find that sound can travel

through wood or metal. In fact, sound

can travel through any solid. You can

perform interesting activities to show

that sound can also travel through

strings. Have you ever made a toy

telephone (Fig. 10.14). Can you say that

sound can travel through strings?

Fig. 10.14 : A toy telephone

We have learnt so far that vibrating

objects produce sound and it is carried

in all directions in a medium. The

medium could be a gas, a liquid or a

solid. How do we hear it?

Fig. 10.15 : Understanding action of an eardrum

10.4 We Hear Sound through

Our Ears

The shape of the outer part of the ear is

like a funnel. When sound enters it, it

travels down a canal at the end of which

there is a thin stretched membrane. It

is called the eardrum. It performs an

important function. To understand what

the eardrum does, let us build a tin-can

model of the eardrum.

Activity 10.10

Take a plastic or tin-can. Cut its

ends. Stretch a piece of rubber

balloon across one end of the can

and fasten it with a rubber band.

Put four or five grains of dry cereal

on the stretched rubber. Now ask

your friend to speak “Hurrey,

Hurrey” from the open end

(Fig.10.15). Observe what happens

to the grain. Why do the grains

jump up and down?

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The eardrum is like a stretched

rubber sheet. Sound vibrations make

the eardrum vibrate (Fig. 10.16). The

eardrum sends vibrations to the inner

ear. From there, the signal goes to the

brain. That is how we hear.

You have already learnt in earlier classes

about the oscillatory motion and its

time period.

The number of oscillations per

second is called the frequency of

oscillation. Frequency is expressed in

hertz. Its symbol is Hz. A frequency of

1 Hz is one oscillation per second. If an

object oscillates 20 times in one second,

what would be its frequency?

You can recognise many familiar

sounds without seeing the objects

producing them. How is it possible?

These sounds must be different to

enable you to recognise them. Have you

ever thought what factors make them

different? Amplitude and frequency are

two important properties of any sound.

Can we differentiate sounds on the basis

of their amplitudes and frequencies?

Loudness and Pitch

Activity 10.11

Take a metallic tumbler and a

tablespoon. Strike the tablespoon

gently at the brim of the tumbler.

Fig. 10.16 : Human ear

Fig. 10.17 : Thermocol ball touching the

vibrating glass tumbler

We must NEVER put a sharp,

pointed or hard thing into our

ear. It can damage the

eardrum. The damaged

eardrum can impair hearing.

10.5 Amplitude, Time Period

and Frequency of a

Vibration

We have learnt that the to and fro motion

of an object is known as vibration. This

motion is also called oscillatory motion.

Eardrum

Inner ear

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Hear the sound produced. Now bang

the spoon on the tumbler and hear

the sound produced again. Is the

sound louder when the tumbler is

struck hard?

Now suspend a small thermocol

ball touching the rim of the tumbler

(Fig. 10.17). Vibrate the tumbler by

striking it. See how far the ball is

displaced. The displacement of the

ball is a measure of the amplitude

of vibration of the tumbler.

Now, strike the tumbler gently

and then a little harder. Compare

the amplitudes of vibrations of the

tumbler in the two cases. In which

case is the amplitude larger?

Loudness of sound is proportional to

the square of the amplitude of the

vibration producing the sound. For

example, if the amplitude becomes

twice, the loudness increases by a

factor of 4. The loudness is expressed

in a unit called decibel (dB). The

following table gives some idea of the

loudness of sound coming from

various sources.

Normal breathing 10 dB

Soft whisper (at 5m) 30 dB

Normal conversation 60 dB

Busy traffic 70 dB

Average factory 80 dB

Above 80 dB the noise becomes

physically painful.

I wonder why my voice

is different from that of

my teacher.

Compare the sound of a baby with

that of an adult. Is there any difference?

Even if two sounds are equally loud,

they differ in some way. Let us see how.

Fig. 10.18 : Frequency determines the

pitch of a sound

The loudness of sound depends on

its amplitude. When the amplitude of

vibration is large, the sound produced

is loud. When the amplitude is small,

the sound produced is feeble.

The frequency determines the

shrillness or pitch of a sound. If the

frequency of vibration is higher we say

that the sound is shrill and has a higher

pitch. If the frequency of vibration is

lower, we say that the sound has a lower

pitch. For example, a drum vibrates with

a low frequency. Therefore, it produces

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a low-pitched sound. On the other

hand, a whistle has a high frequency

and therefore, produces a sound of

higher pitch (Fig. 10.18). A bird makes

a high-pitched sound whereas a lion

makes a low-pitched roar. However, the

roar of a lion is very loud while the

sound of the bird is quite feeble.

Every day you hear the voices of

children and adults. Do you find any

difference in their voices? Can you say

that the frequency of the voice of a child

is higher than that of an adult?

Usually the voice of a woman has

a higher frequency than that of a

man.

10.6 Audible and Inaudible

Sounds

We know that we need a vibrating body

for the production of sound. Can we

hear the sound of all vibrating bodies?

The fact is that sounds of frequencies

less than about 20 vibrations per second

(20 Hz) cannot be detected by the human

ear. Such sounds are called inaudible.

On the higher side, sounds of frequencies

higher than about 20,000 vibrations per

second (20 kHz) are also not audible to

the human ear. Thus, for human ear

the range of audible frequencies is

roughly from 20 to 20,000 Hz.

Some animals can hear sounds of

frequencies higher than 20,000 Hz.

Dogs have this ability. The police use

high frequency whistles which dogs can

hear but humans cannot.

The ultrasound equipment, familiar

to us for investigating and tracking

many medical problems, works at

frequencies higher than 20,000 Hz.

10.7 Noise and Music

We hear different types of sounds

around us. Is the sound always

pleasing? Does a sound sometimes

cause discomfort to you? Some sounds

are pleasant to the ear, whereas some

are not.

Suppose construction work is going

on in your neighbourhood. Are the

sounds coming from the construction

site pleasing? Do you enjoy the sounds

produced by horns of buses and trucks?

Such unpleasant sounds are called

noise. In a classroom, if all the students

speak together, what would the sound

produced be called?

On the other hand you enjoy sounds

from musical instruments. Musical

sound is one which is pleasing to the

ear. Sound produced by a harmonium

is a musical sound. The string of a sitar

also gives out a musical sound. But, if

a musical sound becomes too loud,

would it remain melodious?

10.8 Noise Pollution

You already know about air pollution.

Presence of unwanted gases and

particles in air is called air pollution.

Similarly, presence of excessive or

unwanted sounds in the environment

is called noise pollution. Can you list

some sources of noise pollution? Major

causes of noise pollution are sounds of

vehicles, explosions including bursting

of crackers, machines, loudspeakers etc.

What sources in the home may lead to

noise? Television and transistor radio

at high volumes, some kitchen

appliances, desert coolers, air

conditioners, all contribute to noise

pollution.

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Hearing Impairment

Total hearing impairment, which is rare, is usually from birth itself. Partial

disability is generally the result of a disease, injury or age. Children with

impaired hearing need special care. By learning sign language, such children

can communicate effectively. Because speech develops as the direct result of

hearing, a child with a hearing loss may have defective speech also.

Technological devices for the hearing-impaired have made it possible for such

persons to improve their quality of life. Society can do much to improve the

living environment for the hearing-impaired and help them live normal lives.

What are the Harms of Noise

Pollution?

Do you know that presence of excessive

noise in the surroundings may cause

many health related problems. Lack of

sleep, hypertension (high blood-

pressure), anxiety and many more health

disorders may be caused by noise

pollution. A person who is exposed to a

loud sound continuously may get

temporary or even permanent

impairment of hearing.

Measures to Limit Noise Pollution

To control noise, we must control the

sources of noise. How can this be

achieved? For this, silencing devices

must be installed in air craft engines,

transport vehicles, industrial

machines and home appliances.

How can the noise pollution be

controlled in a residential area?

All noisy operations must be

conducted away from any residential

area. Noise producing industries

should be set up away from such

areas. Use of automobile horns

should be minimised. TV and music

systems should be run at low

volumes. T

rees must be planted along

the roads and around buildings to

cut down on the sounds reaching the

residents, thus reducing the harmful

effects of noise pollution.

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KEYWORDS KEYWORDS

KEYWORDS

AMPLITUDE

EARDRUM

FREQUENCY

hertz (Hz)

LARYNX

LOUDNESS

NOISE

OSCILLATION

PITCH

TIME PERIOD

VIBRATION

VOICE BOX

WIND PIPE

WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT

WHAT YOU HAVE LEARNT

Ü Sound is produced by vibrating objects.

Ü In human beings, the vibration of the vocal

cords produces sound.

Ü Sound travels through a medium (gas, liquid

or solid). It cannot travel in vacuum.

Ü The eardrum senses the vibrations of sound,

It sends the signals to the brain. This process

is called hearing.

Ü The number of oscillations or vibrations per

second is called the frequency of oscillation.

Ü The frequency is expressed in hertz (Hz)

Ü Larger the amplitude of vibration, the louder

is the sound.

Ü Higher the frequency of vibration, the higher

is the pitch, and shriller is the sound.

Ü Unpleasant sounds are called noise.

Ü Excessive or unwanted sounds lead to noise

pollution. Noise pollution may pose health

problems for human beings.

Ü Attempts should be made to minimise noise

pollution.

Ü Plantation on the roadside and elsewhere can

reduce noise pollution.

Exercises

1. Choose the correct answer.

Sound can travel through

(a) gases only (b) solids only

2. Voice of which of the following is likely to have minimum frequency?

(a) Baby girl (b) Baby boy

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3. In the following statements, tick ‘T’ against those which are true, and ‘F’

against those which are false.

(a) Sound cannot travel in vacuum. (T/F)

(b) The number of oscillations per second of a vibrating object is called

its time period. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

4. Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called

__________.

(b) Loudness is determined by the __________ of vibration.

(d) Unwanted sound is called __________ .

(e) Shrillness of a sound is determined by the __________ of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period and

frequency.

6. The sound from a mosquito is produced when it vibrates its wings at an

average rate of 500 vibrations per second. What is the time period of the

vibration?

7. Identify the part which vibrates to produce sound in the following

instruments.

(a) Dholak (b) Sitar (c) Flute

8. What is the difference between noise and music? Can music become noise

sometimes?

9. List sources of noise pollution in your surroundings.

10. Explain in what way noise pollution is harmful to human.

11. Your parents are going to buy a house. They have been offered one on the

roadside and another three lanes away from the roadside. Which house

would you suggest your parents should buy? Explain your answer.

12. Sketch larynx and explain its function in your own words.

13. Lightning and thunder take place in the sky at the same time and at the

same distance from us. Lightning is seen earlier and thunder is heard

later. Can you explain why?

E X E R C I S E S

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Extended Learning — Activities and Projects

1. Visit the music room of your school. You may also visit musicians

in your locality. Make a list of musical instruments. Note down the

parts of these instruments that vibrate to produce sound.

2. If you play a musical instrument, bring it to the class and

demonstrate how you play it.

3. Prepare a list of famous Indian musicians and the instruments

they play.

4. Take a long thread. Place your hands over your ears and get some

one to place this thread round your head and hands. Ask her to

make the thread taut and hold its ends in one hand. Now ask her

to draw her finger and thumb tightly along the thread (Fig. 10.19).

Can you hear a rolling sound like that of a thunder? Now repeat

the activity while another friend stands near both of you. Can he

hear any sound?

Fig. 10.19

5. Make two toy telephones. Use them as shown in Fig. 10.20. Make

sure that the two strings are taut and touch each other. Let one of

you speak. Can the remaining three persons hear? See how many

more friends you can engage in this way. Explain your observations.

Fig. 10.20

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6. Identify the sources of noise pollution in your locality. Discuss with

your parents, friends and neighbours. Suggest how to control noise

pollution. Prepare a brief report and present it in the class.

You can read more on the related topics on the following websites:

l www.physicsclassroom.com/Class/sound/soundtoc.html

l health.howstuffworks.com/hearing.htm

Golconda fort

Did You Know?

Golconda fort, near Hyderabad, is one of the most magnificient

forts in India. It is famous for many engineering and architectural

marvels. One of the marvels is the water supply system. But,

perhaps, more astonishing is a dome near the entrance to the fort.

A hand-clap at a particular point under the dome reverberates

and can be heard at the highest point of the fort, about a kilometre

away. This was devised as a warning system. If a guard saw a

suspicious movement outside the fort, he clapped at the particular

point under the dome, and the army inside the fort was alerted to

the danger of the approaching enemy.

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